1. Field of the Invention
The present invention relates to the heat treatment of semiconductor wafers and more specifically to an apparatus for heat treating of layers formed over semiconductor wafers.
2. Description of the Related Art
Semiconductor devices are used extensively in today""s marketplace. In order to meet the increased demand for devices employing semiconductor devices, techniques to fabricate semiconductor devices and apparatus used to fabricate semiconductor devices must increase in efficiency.
Semiconductor devices are formed from silicon wafers containing various circuitry defining the semiconductor device. During the formation of the circuitry on the silicon wafer, the wafers are subjected to various processes to enhance the quality of the layers formed over the wafers. As is well known, layers formed over wafers in the form of thin films must be cured in order to carry out thermolytic reactions and/or remove solvents. Typically, wafers are cured in curing modules. However, current cure modules suffer from non-uniform heating of wafers and thermal leakage within the module. These problems serve to increase the cure time of a wafer and the overall fabrication time for a semiconductor wafer.
FIG. 1 shows a cure module 10 in accordance with the prior art. The cure module 10 shown in FIG. 1 includes a heating plate 12 and a cooling chamber 20. A semiconductor wafer 11 is placed within a chamber 14 of the cure module 10 through a door 24 that is connected to an arm 24a. The semiconductor wafer 11 is set on lift pins 22 at lift points 22a in the cure module 10 as shown. During a heating operation, the wafer 11 is lowered onto heating plate standoffs 21. The heating plate standoffs 21 are integrally formed with the heating plate 12. After the wafer 11 is placed on the heating plate standoffs 21, the wafer 11 will be held in place over the heating plate 12 with alignment pins 19, as shown in FIG. 1.
As can be seen from FIG. 1, the lift pins 22, alignment pins 19 and heating plate standoffs 21 are in thermal contact with both the heating plate 12 and the wafer 11. The lift pins 22 pass through the heating plate 12 and the alignment pins 19 and heating plate standoffs 21 are in thermal contact with the heating plate 12. Thus, the temperature uniformity of the heating plate 12 will be reduced due to thermal losses from the lift pins 22, alignment pins 19 and heating plate standoffs 21.
The temperature losses in the hot plate due to the alignment pins 19, the lift pins 22 and the heating plate standoffs 21 will cause thermal discontinuities in the wafer 11 as shown by the contact regions 13 in FIG. 1. The thermal discontinuities at the contact regions 13 occur where the wafer is in thermal contact with the alignment pins 19, heating plate standoffs 21 and the lift pins 22. Thus, the non-contact regions 15 are at a lower temperature than the contact regions 13. The thermal discontinuities and the resulting non-uniform heat treatment of the wafer 11 depicted by the non-contact regions 15 and the contact regions 13 will cause undesirable changes in the film properties on the surface of the wafer which may potentially affect the performance of a semiconductor device formed from the wafer.
After the wafer 11 is heated, the wafer 11 must be cooled so that organic components of the film will not oxidize on the surface of the wafer 11 after the wafer 11 is removed from the chamber 14 and exposed to air. The wafer 11 is cooled by first raising it on the lift pins 22 from the heating plate 12 towards a diffusion plate 18. The diffusion plate 18 acts as a shower head for the cooling medium of the cure module 10. The wafer 11 is commonly cooled with a water cooled (H2O) nitrogen gas (N2) flowing through the cooling chamber 20. The nitrogen gas is dispensed through the diffusion plate 18 onto the wafer 11. While the wafer 11 is being heated, heat will rise from the heating plate 12 to the diffusion plate 18, increasing the surface temperature of the diffusion plate 18. The increased temperature of the diffusion plate 18 raises the temperature of the nitrogen gas as it exits through the diffusion plate 18. The higher temperature of the nitrogen gas increases the cooling time of the wafer 11, thereby decreasing the overall efficiency of the curing process.
The diffusion plate 18 also affects the flow rate of the nitrogen gas into the cure module 10. The diffusion plate 18 may contain particles (i.e., due to particle condensation from volatile organic material leaving the wafer surface during the bake process) which may become dislodged during the nitrogen cooling operation. Thus, in order to prevent the dislodging of particles, the nitrogen gas must be passed at a slower rate, thereby decreasing the cooling rate of the wafer 11 and the overall efficiency of the cure module 10.
The seals 23 also make it difficult to maintain a low oxygen environment at high temperatures within the cure module and make vacuum baking impossible. A low oxygen environment is desirable to eliminate the possibility of film oxidation within the chamber 14 during the curing process. Further, during heating the seals 23 necessarily increase in temperature along with the lift pins 22. As can be appreciated, when the seals 23 are constantly exposed to high temperatures, their sealing capabilities are decreased and their useable lifetime is reduced. Therefore, this increases the possibility of oxygen entering the chamber 14 while the wafer is being cured, along with the possibility of oxidation of the film on the wafer and the generation of particulates by the aging seals.
Wafers are loaded into the cure module 10 through a door mechanism 24. The door mechanism 24 is pivoted into place against the cure module 10 with a door mechanism arm 24a. As the door mechanism arm 24a pivots the door mechanism 24 into place against the cure module 10, both the door mechanism 24 and door mechanism arm 24a release particulates outside the chamber. These particulates may fall onto wafers as they are being moved into other processing chambers (not shown) and into the cure module 10. These particulates are a possible cause of defects in a semiconductor device eventually formed with the wafer.
Lift pin through holes 25 also cause thermal discontinuities on the wafer 11. The lift pin through holes 25 represent voids in the heating plate 12, or areas where there is no source of heat emanating from the heating plate 12. These voids cause uneven heating of the wafer 11, which will result in thermal discontinuities.
In view of the foregoing, there is a need for a cure module which is configured to efficiently cure films formed over silicon wafers. There is also a need for modules that can cure formed films in a uniform manner and that avoid the introduction of thermal discontinuities that can affect the performance of the cured films, such as, spin-on glass (SOG), dielectrics including low-K dielectrics, and any other type of applied film.
Broadly speaking, the present invention fills these needs by providing a curing module with an inverted hot plate. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a curing module for use in fabricating semiconductor wafers is disclosed. The cure module comprises a chamber having a bottom plate and a housing. The housing has a top surface and sidewalls that are configured to engage with the bottom plate. The top surface, sidewalls and the bottom plate define the chamber. The cure module also has a heating plate within the chamber and that is adjacent to the top surface of the housing.
In another embodiment, a method for making a cure module is disclosed. A housing is generated having a top surface and sidewalls. A bottom plate that is configured to engage with the sidewalls of the housing to create a chamber is then generated. After the bottom plate is generated, a heating plate is attached to the top surface of the housing within the chamber.
In yet another embodiment, a module for curing films on a semiconductor wafer is disclosed. In this embodiment, the module comprises a bottom plate and a heating plate which is located above the bottom plate.
In another embodiment an apparatus for curing wafers is disclosed. The apparatus comprises a cooling bottom plate having cooling fins located on the cooling bottom plate. The cooling fins cool the bottom plate such that the bottom cooling plate cools the wafer during a curing operation. The cure module also has a housing which sealingly engages with the cooling bottom plate and a heating plate located inside the housing. The heating plate is above the cooling bottom plate and the heating plate heats films on semiconductor wafers during a curing operation.
The many advantages of the present invention should be recognized. A cure module can now cure films on semiconductor wafers without introducing thermal discontinuities. The location of the heating plate above the cooling plate allows components necessary for the operation of the cure module, (e.g. lift pins and alignment pins) to be located in the bottom cooling plate portion of the cure module. Therefore, as these components are in contact with wafers, they will not introduce hot spots which cause thermal discontinuities. Also, the heating plate has a uniform surface due to the lack of lift pin through holes, which allows for uniform heating of a wafer.
The chamber for the present invention comprises the heating plate and the cooling plate. All other major components (e.g. O-rings, housing and lift pin positioners) are located outside of the chamber. As a result, these components are not subjected to the temperature fluctuations occurring inside the chamber, (i.e., heating and cooling operations) which decrease the life span and reliability of these components. Hence, the cure module is more cost efficient in that the components last much longer and do not necessitate replacement as often as components in prior art cure modules. In addition, no cooling water is needed.
The present invention has the added advantage of having the heating operation being done from above. This decreases both the cure times and the overall costs of curing films on wafers. Heat from the heating plate will not convect upward to warm the cooling plate, thus, the cooling plate will remain at a reduced temperature and films being cooled by the plate without the use of cooling water (as in the prior art) which greatly simplifies module design and maintenance.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.